Regenerative ring resonator

ABSTRACT

A laser includes a regenerative ring resonator that includes a discharge chamber having electrodes and a gain medium between the electrodes for producing a laser beam; a partially-reflective optical coupler, and a beam modification optical system in the path of the laser beam. The beam modification optical system transversely expands a profile of the laser beam such that the near field laser beam profile uniformly fills each aperture within the laser and such that the regenerative ring resonator remains either conditionally stable or marginally unstable when operating the laser at powers that induce thermal lenses in optical elements inside the regenerative ring resonator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.12/413,341, filed Mar. 27, 2009 and also claims the benefit of U.S.Provisional Application No. 61/164,297, filed Mar. 27, 2009. Both ofthese applications are herein incorporated by reference in theirentirety.

TECHNICAL FIELD

The disclosed subject matter relates to a recirculating ring resonatorof a high power laser system such as a gas discharge laser.

BACKGROUND

Gas discharge lasers are used in photolithography to manufacturesemiconductor integrated circuits. As semiconductor manufacturing hasprogressed to requiring smaller and smaller feature sizes (that is, theminimum feature size used to fabricate the integrated circuit), thedesign and performance of these lasers has improved. For example, gasdischarge lasers have been redesigned to provide shorter wavelength andnarrower bandwidth to support higher resolution, to provide higherpowers to enable higher throughput, and to stabilize performanceparameters such as dose, wavelength, and bandwidth.

Excimer lasers are one type of gas discharge laser used inphotolithography that can operate in the ultraviolet (UV) spectralregion at high average output power to generate nanosecond pulses atreduced spectral bandwidth.

In some cases, these lasers are designed with a dual chamber designhaving first and second chambers to separate the functions of providingnarrow spectral bandwidth and generating high average output pulseenergy. The first chamber is called a master oscillator (MO) thatprovides a seed laser beam and the second chamber is called a poweramplifier (PA), a power oscillator (PO), or a power ring amplifier (PRA)and receives the seed laser beam from the MO. The MO chamber enablesfine tuning of parameters such as the center wavelength and thebandwidth at relatively low output pulse energies. The power amplifierreceives the output from the master oscillator and amplifies this outputto attain the necessary powers for output to use in photolithography.The dual chamber design can be referred to as a MOPA, MOPO, or MOPRA,depending on how the second chamber is used.

SUMMARY

In some general aspects, a laser includes a regenerative ring resonatorthat includes an amplifier discharge chamber having electrodes and again medium between the electrodes for producing a laser beam; anoptical coupler, and a beam modification optical system in the path ofthe laser beam. The optical coupler is partially reflective so that atleast a portion of the laser beam impinging on the optical coupler fromthe amplifier discharge chamber is reflected back through the amplifierdischarge chamber and at least a portion of the laser beam impinging onthe optical coupler from the amplifier discharge chamber is transmittedthrough the optical coupler. The beam modification optical systemtransversely expands a profile of the laser beam such that the nearfield laser beam profile uniformly fills each aperture within the laserand such that the regenerative ring resonator remains eitherconditionally stable or marginally unstable when operating the laser atpowers that induce thermal lenses in optical elements inside theregenerative ring resonator.

The beam modification optical system reduces variations in output beamsize that result from operation of a gas discharge laser amplifier overa range of average powers, some of which can be high.

Implementations can include one or more of the following features. Forexample, the beam modification optical system can be between the opticalcoupler and a beam turning optical element placed on a side of thedischarge chamber opposite to a side facing the optical coupler. Thebeam modification optical system can be configured to cause the laserbeam exiting the regenerative ring resonator to have the same or alarger size of the transverse profile than the size of the transverseprofile of the laser beam entering the regenerative ring resonator.

The beam modification optical system can be configured to impart anegative curvature to the wavefront of the laser beam circulating withinthe regenerative ring resonator. The beam modification optical systemcan negatively alter the curvature along a transverse direction. Thebeam modification optical system can include a highly reflective mirror.The highly reflective mirror can be convex. The convex highly reflectivemirror can have a radius of curvature of between about 50 m and about170 m.

The laser can also include a beam turning optical element external tothe discharge chamber and in the path of the laser beam on a side of thedischarge chamber that is opposite to a side that faces the opticalsystem.

The regenerative ring resonator can remain marginally unstable if thesize of the transverse profile of the laser beam increases as the laserbeam travels through a portion of the regenerative ring resonator butthe laser beam transverse profile size does not exceed the transversesize of any of the optical components within the regenerative ringresonator before being decoupled out of the regenerative ring resonatorthrough the optical coupler.

The beam modification optical system can include a set of prisms. Theprism set can include first, second, and third prisms configured andarranged so that the first and third prisms reduce the transverse sizeof the profile of the laser beam travelling along a first directionthrough the beam modification optical system, and the third and secondprisms increase the transverse size of the profile of the laser beamtravelling along a second direction through the beam modificationoptical system. The one or more first, second, and third prisms can beadjusted so that the transverse size of the profile of the laser beamtravelling along the second direction through the beam modificationoptical system and exiting the prism set is greater than the transversesize of the profile of the laser beam travelling along the firstdirection from the optical coupler to the prism set.

The laser beam output from the regenerative ring resonator can have anaverage irradiance of at least about 5 W/cm². In other implementations,the laser beam output from the regenerative ring resonator can have anaverage irradiance of at least about 10 W/cm². In some implementations,the peak irradiance of the laser beam output from the regenerative ringresonator can be less than 30 mJ/cm².

In other general aspects, a laser beam of an electric discharge gaslaser is modified by directing the laser beam through an optical couplerof a regenerative ring resonator; directing the laser beam that passesthrough the optical coupler through a discharge chamber and back to theoptical coupler such that at least some of the light impinging on theoptical coupler from the discharge chamber is reflected back through thedischarge chamber and at least some of the light impinging on theoptical coupler from the discharge chamber is transmitted through theoptical coupler; and transversely expanding a profile of the laser beamsuch that the near field laser beam profile uniformly fills eachaperture within the laser and such that the regenerative ring resonatorremains either conditionally stable or marginally unstable whenoperating the laser at powers that cause thermal lensing of elementswithin the regenerative ring resonator.

Implementations can include one or more of the following features. Forexample, the profile of the laser beam can be transversely expanded bycompressing a profile of the laser beam by passing the laser beam fromthe optical coupler through first and third prisms of a prism set beforedirecting the laser beam through the discharge chamber; and expanding aprofile of the laser beam after it has passed through the dischargechamber by passing the laser beam through the third prism and through asecond prism of the prism set before reaching the optical coupler. Thelaser beam profile can be expanded by expanding the laser beam profileto a size that is larger than the profile of the laser beam input to theprism set.

The regenerative ring resonator can remain marginally unstable if thesize of the transverse profile of the laser beam increases as the laserbeam travels through at least a portion the regenerative ring resonatorbut the laser beam transverse profile size does not exceed thetransverse size of any of the optical components within the regenerativering resonator before being decoupled out of the regenerative ringresonator through the optical coupler.

The profile of the laser beam can be transversely expanded by impartinga negative curvature to the wavefront of the laser beam circulatingwithin the regenerative ring resonator.

In other general aspects, a regenerative ring resonator that is in thepath of a laser beam includes a discharge chamber having electrodes anda gain medium between the electrodes; an optical coupler that ispartially reflective so that at least some of the light impinging on theoptical coupler from the discharge chamber is reflected back through thedischarge chamber and at least some of the light impinging on theoptical coupler from the discharge chamber is transmitted through theoptical coupler; and a beam modification optical system in the path ofthe laser beam. The beam modification optical system is configured totransversely expand a profile of the laser beam such that the near fieldlaser beam profile uniformly fills each aperture within the resonatorand such that the regenerative ring resonator remains eitherconditionally stable or marginally unstable when operating the laser atpowers that cause thermal lensing of elements inside of the regenerativering resonator.

Implementations can include one or more of the following features. Thebeam modification optical system can be between the optical coupler anda beam turning optical element placed on a side of the discharge chamberopposite to a side facing the optical coupler.

The beam modification optical system can be configured to impart anegative curvature to the wavefront of the laser beam circulating withinthe regenerative ring resonator. The beam modification optical systemcan include a highly reflective convex mirror.

The regenerative ring resonator can remain marginally unstable if thesize of the transverse profile of the laser beam increases as the laserbeam travels through at least a portion of the regenerative ringresonator but the laser beam transverse profile size does not exceed thetransverse size of any of the optical components within the regenerativering resonator before being decoupled out of the regenerative ringresonator through the optical coupler.

The beam modification optical system can include a set of prisms. Theprism set can include first, second, and third prisms configured andarranged so that the first and third prisms reduce the transverse sizeof the profile of the laser beam travelling along a first directionthrough the beam modification optical system, and the third and secondprisms increase the transverse size of the profile of the laser beamtravelling along a second direction through the beam modificationoptical system. The one or more first, second, and third prisms can beadjusted so that the transverse size of the profile of the laser beamtravelling along the second direction and exiting the prism set isgreater than the transverse size of the profile of the laser beamtravelling along the first direction from the optical coupler to theprism set.

DRAWING DESCRIPTION

FIG. 1 is a block diagram of a high average-power laser system providinginput to a lithography machine;

FIG. 2A is a plan view of a power ring amplifier of the laser system ofFIG. 1;

FIG. 2B is a plan view of a beam reverse of the power ring amplifier ofFIG. 2A;

FIG. 2C is a plan view of a chamber window of a gas discharge chamber ofthe power ring amplifier of FIG. 2A;

FIG. 2D is a plan view of a beam modification optical system of thepower ring amplifier of FIG. 2A;

FIG. 3 is a side view of the power ring amplifier of FIG. 2A;

FIG. 4 is a perspective view of the beam modification optical system ofFIG. 2D;

FIG. 5 is a perspective view of a first implementation of the beammodification optical system mounted to a housing;

FIG. 6 is a plan view of the first implementation of the beammodification optical system mounted to the housing;

FIGS. 7A and 7B are, respectively, front and back perspective views of ahighly reflective mirror mounted to a mirror mount that is attached tothe beam modification optical system housing of FIGS. 5 and 6;

FIG. 8A is a plan view of the highly reflective mirror of FIG. 7A;

FIGS. 8B and 8C are orthogonal side views of the highly reflectivemirror of FIG. 8A;

FIG. 9 is a perspective view of a second implementation of the beammodification optical system mounted to a housing;

FIG. 10 is a plan view of the second implementation of the beammodification optical system mounted to the housing;

FIG. 11A is a perspective view of a highly reflective mirror mounted toan adaptive mirror mount that is attached to the beam modificationoptical system housing of FIGS. 9 and 10;

FIGS. 11B-11D are perspective views of the mount for the highlyreflective mirror of FIG. 11A;

FIG. 12A is a plan view of the highly reflective mirror of FIG. 11A;

FIGS. 12B and 12C are orthogonal side views of the highly reflectivemirror of FIG. 12A;

FIG. 13A is a plan view of another implementation of the beammodification optical system of the power ring amplifier of FIG. 2A;

FIG. 13B is a plan view of a detail of the beam modification opticalsystem of FIG. 13A;

FIG. 14 is a graph of the signal amplitude versus distance from thecenter of the laser beam taken for different curvatures of the highlyreflective mirror; and

FIGS. 15A-15C are optical diagrams showing, respectively, a negativecurvature wavefront, a positive curvature wavefront, and a zerocurvature wavefront.

DESCRIPTION

Referring to FIG. 1, a high average-power repetitively-pulsed lasersystem 100 produces a high power repetitively-pulsed laser beam 102 thatis delivered through a beam delivery unit 104 to a lithography machine106. The laser system 100 includes a master oscillator (MO) 108 thatprovides a seed laser beam 110 to a power ring amplifier (PRA) 112(having a discharge chamber with a regenerative ring resonator). Themaster oscillator 108 enables fine tuning of parameters such as thecenter wavelength and the bandwidth at relatively low output pulseenergies. The power ring amplifier 112 receives the output from themaster oscillator and amplifies this output to attain the necessarypowers in the laser beam 102 for output to use in the lithographymachine 106.

The master oscillator 108 includes a discharge chamber 114 having twoelongated electrodes, a laser gas, and a fan for circulating the gasbetween the electrodes, and a laser resonator is formed between a linenarrowing module 116 on one side of the discharge chamber 114 and anoutput coupler 118 on a second side of the discharge chamber 114. Theline narrowing module 116 can include a diffractive optic such as agrating that finely tunes the spectral output of the discharge chamber114. The master oscillator 108 also includes a line center analysismodule 120 that receives an output from the output coupler 118 and abeam modification optical system 122 that modifies the size and/or shapeof the laser beam as needed. The laser gas used in the discharge chambercan be any suitable gas for producing a laser beam at a requiredwavelength and bandwidth, for example, the laser gas can be argonfluoride (ArF), which emits light at a wavelength of about 193 nm,krypton fluoride (KrF), which emits light at a wavelength of about 248nm, or xenon chloride (XeCl), which emits light at a wavelength of about351 nm.

The power ring amplifier 112 includes a beam modification optical system124 that receives the seed laser beam 110 from the master oscillator 108and directs the laser beam through a power ring amplifier dischargechamber 126, and to a beam turning optical element 128 where thedirection of the laser beam is modified so that it is sent back into thedischarge chamber 126 to form a circulating path that is also referredto as a regenerative ring resonator. The power ring amplifier dischargechamber 126 includes a pair of elongated electrodes, a laser gas, and afan for circulating the gas between the electrodes. The seed laser beam110 is amplified by repeatedly passing through the power ring amplifier112. The optical system 124 provides a way (for example, an opticalcoupler such us a partially-reflecting mirror 202, discussed below) toin-couple the seed laser beam 110 and to out-couple a portion of theamplified radiation from the ring resonator to form an output laser beam130. The output laser beam 130 is directed through a bandwidth analysismodule 132, then through a pulse stretcher 134, where each of the pulsesof the output laser beam 130 is stretched, for example, in an opticaldelay unit, to adjust for performance properties of the laser beam thatimpinges the lithography machine 106. The laser beam 102 that exits thepulse stretcher 134 can be directed through an automatic shutter 136before entering the beam delivery unit 104.

The laser system 100 also includes a control system 138 coupled to themaster oscillator 108 and to the power ring amplifier 112 torcontrolling the pulse energy and accumulated dose energy output of thesystem 100 at pulse repetition rates of between about 4000 and 12,000 Hzor greater. The control system 138 provides repetitive triggering of thedischarges in the chamber of the master oscillator 108 and thedischarges in the chamber of the power ring amplifier 112 relative toeach other with feedback and feed-forward control of the pulse and doseenergy. The high power repetitively-pulsed laser beam 102 can have anaverage output power of between a few watts and hundreds of watts, forexample, from about 40 W to about 200 W. The irradiance (that is, theaverage power per unit area) of the laser beam 102 at the output can beat least about 5 W/cm² or at least about 10 W/cm².

Referring also to FIGS. 2A-4, the power ring amplifier 112 is designedas a regenerative ring resonator. The seed laser beam 110 from themaster oscillator 108 is directed to a folding mirror 200 of the beammodification optical system 124. The folding mirror 200 reflects thebeam and directs it through an optical coupler, which is a partiallyreflecting mirror (and is sometimes referred to as an input/outputcoupler) 202, which is the entrance to the ring resonator and then to ahighly reflective mirror 204. The mirror 204 is highly reflective if itsreflectivity is greater than about 90% at or near the center wavelengthof the laser beam for the desired polarization at the angle(s) ofincidence used.

The highly reflective mirror 204 reflects the laser beam 110 through afirst prism 206 and a third prism 208 that act together to compress thelaser beam 110 horizontally to substantially match the transverse sizeof the gain medium, which is typically less than a few millimeters (mm)in high repetition-rate discharge-pumped excimer lasers. The third prism208 aligns the laser beam 110 with a right chamber window 210 and adesired light path through the chamber 126, through a left chamberwindow 212, and to the beam turning optical element 128. From the beamturning optical element 128, the laser beam returns to the left chamberwindow 212, passes through the chamber 126 and the right chamber window210, and then through the third prism 208, which shifts the laser beamto a second prism 214, which shifts the laser beam to the input/outputcoupler 202. The third and second prisms 208 and 214 acting together inthis fashion magnify the beam exiting the chamber window 210 to matchthe transverse size of the incoming laser beam 110 and/or the desiredhorizontal size of the output laser beam 130. The beam impinging uponthe input/output coupler 202 can be transmitted through the coupler 202to form an amplified laser beam 130 that is directed toward thebandwidth analysis module 132. The input/output coupler 202 is partiallyreflective, for example, 20% reflective, such that at least some of thelight impinging upon the input/output coupler 202 can be reflected backto the discharge chamber 126 through the optical system 124, providingregenerative feedback.

The first and third prisms 206 and 208 are positioned and arrangedrelative to each other so that they reduce the size of the transverseprofile of the laser beam 110 travelling along a first direction throughthe wavefront modification optical system 124. That is, the first andthird prisms 206 and 208, in combination and in the geometricconfiguration shown, de-magnify the horizontal size of the laser beamthat travels along the first direction, which is from the highlyreflective mirror 204 toward the right chamber window 210, tosubstantially match the transverse size of the discharge plasma andeffectively utilize the laser gain. The third and second prisms 208 and214 are positioned and arranged relative to each other so that theyincrease the size of the transverse profile of the laser beam 130travelling along a second direction through the wavefront modificationoptical system 124. That is, the third and second prisms 208 and 214, incombination and in the geometric configuration shown, magnify thehorizontal size of the laser beam that travels along the seconddirection, which is from the right chamber window 210 to theinput/output coupler 202, to match the transverse extent of the incominglaser beam 110 and/or the desired horizontal size of the output laserbeam 130.

The beam turning optical element 128 is an optical system made of one ormore precision devices each having precision optical materials such as,for example, materials having a crystalline structure such as calciumfluoride (CaF2). Additionally, the beam turning optical element 128 hasprecision optically finished faces. The beam turning optical element 128can be any combination of one or more optical devices that receive alight beam and change a direction of the light beam so that it istransmitted back into the discharge chamber 126. For example, the beamturning optical element 128 can be a prism having two reflectingsurfaces, as shown in FIGS. 2A and 2B. As another example, the beamturning optical element 128 can include a plurality of mirrors arrangedto reflect a beam back into the discharge chamber 126.

The input/output coupler 202 is a partially reflective mirror, forexample, with between about 10% to about 60% reflectivity back into thechamber 126, thus forming an oscillation cavity that allows for laserpulse intensity build up during the oscillation through the excited gasgain medium between the electrodes within the chamber 126 during theelectrical discharge.

The optical components (such as the coupler 202, highly reflectivemirror 204, prisms 206, 208, 214, the highly reflective mirror 204, thechamber windows 210, 212, and the beam turning optical element 128) ofthe power ring amplifier 112 are typically crystalline structures thatare able to transmit very high pulse energy laser pulses at very shortwavelengths with minimal losses, for example, 193 nm or 248 nm. Forexample, these components can be made of calcium fluoride (CaF2),magnesium fluoride (MgF2), or fused silica.

In summary, the optical components (coupler 202, highly reflectivemirror 204, and prisms 206, 208, and 214) of the beam modificationoptical system 124 direct the seed laser beam 110 through the chamber126, where the laser beam is amplified, and then passed to the beamturning optical element 128, which directs the laser beam back throughthe chamber 126 where the laser beam is further amplified and at leastsome of the further amplified laser beam is passed through the coupler202 to exit the power ring amplifier 112 as the output laser beam 130while at least some of the laser beam is reflected by the coupler 202back into the ring resonator for further amplification.

The following discussion uses the terms “beam profile,” “near field,”“far field,” “system aperture,” and “aperture” to describe some of theoptical effects noticed within the power ring amplifier 112.

The term “aperture” is a hole, structure, or opening through which lighttravels. More specifically, the aperture of an optical system is anopening that determines the cone angle of a bundle of rays that come toa focus in the image plane. The power ring amplifier 112 can have manyopenings or structures that limit the ray bundles. For example, thesestructures may be the edge of a lens or a mirror, an opening in anotherwise opaque body, or a ring or other fixture that holds an opticalelement in place, or may be a special element such as a prism placed inthe optical path to limit the light admitted by the system. In general,these structures are called stops, and the aperture stop is the stopthat determines the ray cone angle, or equivalently the brightness, atan image point.

The laser system 100 has a defined aperture stop at the output of thepulse stretcher 134; the defined aperture stop at the output of thelaser system 100 is also referred to as the “system aperture.”

The term “beam profile” is the distribution of energy in position acrossa direction that is transverse to the beam propagation direction. The“near field” beam profile refers to the distribution of energy inposition across an aperture close to or very near to the aperture. The“far field” beam profile is the distribution of energy in positionacross an aperture at a plane far away from the aperture. Practically,the distribution of energy in position far enough away from the systemaperture is entirely dominated by distribution in angle at the systemaperture. Thus, if a light beam had zero divergence, then thedistribution of energy in position will be the same at aperture and alsovery far away from the aperture. If a light beam has non-zerodivergence, then there is a distance beyond which the spreading due todivergence (roughly angle×distance) contributes much more to thedistribution of energy in position than did the initial (near-field)distribution.

The output laser beam 130 can suffer from narrowing of the horizontalnear-field profile such that the laser beam 130 exiting the laser beammodification optical system 124 has a smaller horizontal profile thanthe horizontal profile of the laser beam 110 entering the beammodification optical system 124. This narrowing can be caused bywavefront variations when the operating duty cycle is such that theaverage power incident upon or transmitted through the opticalcomponents of the power ring amplifier 112 is very high. Such wavefrontvariations can result from heating of the optical components (such as,for example, the chamber windows 210, 210) due to absorption of afraction of the optical power circulating in the power ring amplifier112, which induces positive thermal lenses in these optical components.

The beam modification optical system 124 is designed to transverselyexpand the beam profile of the laser beam exiting the power ringamplifier 112 relative to the laser beam wavefront entering the powerring amplifier 112. The term “transverse” can be any direction that isperpendicular to an optical axis (which is also referred to as alongitudinal direction) of the laser beam 110. Thus, the beam profile istransversely expanded if the profile is expanded along a direction thatis perpendicular to the beam's optical axis.

In some implementations, this expansion system can be a system thatnegatively alters the wavefront curvature of the laser beam so that thedivergence of the laser beam increases to offset the horizontalnear-field profile narrowing. Thus, the wavefront curvature imparted tothe beam is more negative relative to the laser beam coming out of theother elements inside of the ring amplifier 112 so as to compensate, forexample, for a positive thermal lens in one or more of the opticalcomponents of the ring amplifier 112 such as a prism or the chamberwindows. In the near field, the size of the laser beam is notappreciably larger; but after the laser beam has propagated around thepower ring amplifier 112 a few times, the beam's transverse dimensioncan increase appreciably.

In other implementations, this expansion system can be a system thatsimply magnifies (for example, using refraction across planar surfacesof sets of prism pairs) the beam profile so that the transverse spotsize of the laser beam is broadened in the near-field to offset thehorizontal near-field profile narrowing. In this case, the imbalancedprism pairs magnify the laser beam and increase its transverse dimensionimmediately at the exit face of the last prism of the wavefrontexpansion system. Moreover, the imbalanced prism pairs can also reducethe divergence of the beam even though they serve to expand or broadenthe beam in the near-field.

In either case, the expansion system keeps the near field laser beamfrom collapsing so that the near field laser beam uniformly fills all ofthe apertures within the laser system 100 including apertures within thepower ring amplifier 112 and the system aperture while preserving otherproperties (such as a relatively low horizontal divergence) of theoutput laser beam 130, as discussed in greater detail below.

Thus, the beam modification optical system 124 can be designed with anegative wavefront curvature system that negatively alters the curvatureof the wavefront of the laser beam 130 exiting the beam modificationoptical system 124 relative to the laser beam 110 entering the beammodification optical system 124. In this way, the laser beam 130 has thesame or a larger transverse (for example, horizontal) profile than thetransverse (for example, horizontal) profile of the laser beam 110entering the beam modification optical system 124. Additionally, thewavefront curvature is preferably altered such that the regenerativering resonator (the PRA 112) does not become a stable resonator butremains either conditionally stable or marginally unstable whenoperating the laser at powers that induce thermal lenses in opticalelements inside of the regenerative ring resonator.

The ring resonator is conditionally stable if the laser beam transverseprofile remains substantially constant at a particular location insidethe resonator after an infinite number of passes through the resonatorand the laser beam transverse profile does not ever exceed thetransverse size of any of the optical components within the resonatoreven after an infinite number of passes through the resonator. The ringresonator is marginally unstable if the laser beam transverse profileincreases in size as it travels through the resonator but does not everexceed the transverse size of any of the optical components within theresonator before being decoupled out of the resonator. For example, ifthe laser pulse duration is about 20 ns, and it takes about 4-5 ns forlight complete a single round trip through the regenerative ringresonator, then the laser pulse will be decoupled out of the resonatorafter about 4-5 round trips through the regenerative ring resonator. Toput it another way, the laser beam transverse profile remains fullysupported by optical components within a resonator even though the sizeof the transverse profile stays the same or increases while travelingthrough the resonator. In a stable (but not conditionally stable)resonator, the laser beam transverse profile size can decrease atvarious stages as it passes through the resonator and the laser beamtransverse profile size does not ever exceed the transverse size of anyof the optical components within the resonator no matter how many timesit passes through the resonator.

Referring to FIG. 15A, a wavefront has a “negative” curvature if acenter 1500 of the wavefront 1505 is pointing along a propagationdirection 1510 of the laser beam, that is, the wavefront center 1500 isadvanced when compared to an edge 1515 of the wavefront 1505. Referringto FIG. 15B, a wavefront has a “positive” curvature if a center 1520 ofthe wavefront 1525 is pointing opposite to a propagation direction 1530of the laser beam, that is, the wavefront center 1520 is retarded whencompared to a wavefront edge 1535. Therefore, the beam modificationoptical system 124 negatively alters the curvature of the wavefront ofthe laser beam 130 relative to the laser beam 110 if the center of thewavefront of the laser beam 130 is advanced farther than the center ofthe wavefront of the laser beam 110 relative to the respective wavefrontedges. Referring to FIG. 15C, a wavefront has “zero” curvature if acenter 1540 of a wavefront 1545 aligns, transversely to a propagationdirection 1550, with a wavefront edge 1555.

Narrowing of the transverse output energy distribution (also referred toas “beam narrowing”) can occur in the vertical direction but the impactof the narrowing may not be significant since the thermal gradients canbe lower in the vertical direction for many practical high-repetitionrate excimer laser systems (in particular, the mode side in the verticaldirection can be about ten times as large as the mode size in thehorizontal direction). Thus, the beam narrowing could be corrected in ageneral transverse direction, which is any direction that isperpendicular to the path that the laser beam travels (that is, theoptical axis), and therefore can be the horizontal direction or thevertical direction if the beam profile is aligned with the horizontaland vertical direction.

The negative wavefront curvature system can be integrated with any ofthe components of the optical system 124, for example, it can be amodification to the design or location of one or more components of theoptical system 124.

The negative wavefront curvature system can be implemented by modifyingthe highly reflective mirror 204 to have a curved (convex) reflectivesurface. In prior designs, the highly reflective mirror 204 issubstantially flat (to an accuracy within a wavelength of the light thatis reflected from the mirror 204) so that it would not impart anysignificant wavefront change (such as, for example, a change incurvature to the wavefront) to the reflected laser beam. In the negativewavefront curvature system, the highly reflective mirror 204 has aslightly convex shape to impart a small amount of negative curvature tothe wavefront to the wavefront of the laser beam reflected by the mirror204. The highly reflective mirror 204 can be curved either bymanufacturing a mirror to have a permanent convex profile in thetransverse direction of interest (as described with respect to FIGS.5-8C) or by bending a flat mirror to have a convex shape using a bendingdevice (as described with respect to FIGS. 9-12C).

Referring to FIGS. 5-8C, the beam modification optical system 124 isarranged on a support 500 that is within a sealed housing 502. In thisimplementation, the highly reflective mirror 504 is manufactured to havea permanent convex profile in the horizontal direction on its reflectivesurface 506 and is mounted to a mirror mount 508 that is arranged on thesupport 500. The highly reflective mirror 504 is made upon a crystallinesubstrate or other substrate that is robust against exposure to highaverage powers at very short wavelengths, for example, at wavelengths of193 nm or 248 nm. For example, the highly reflective mirror 504 can beformed upon a calcium fluoride (CaF2) or magnesium fluoride (MgF2)substrate. The clear aperture 510 of the mirror 504 is shown in dashedlines in FIG. 8A. The convex profile can be any convex shape, forexample, it can be in the shape of an arc of a cylinder. Thereflectivity of the surface 506 can be greater than 94% (for example,from about 94% to about 97%) for a ray striking the surface at 45° andat a wavelength of about 193 nm in the desired polarization state. Asshown in FIGS. 8A-C, in this design, the highly reflective mirror 504has a convex profile in only the horizontal direction 800 such that thelaser beam profile along the vertical direction 850 is substantiallyunchanged upon reflection from the mirror 504. The convex profile can besuch that the mirror 504 has a radius of curvature along the horizontaldirection of between about 50 m and 170 m. To put it another way, theconvex profile is such that mirror 504 has a surface sag of betweenabout 300 nm to about 1000 nm for a 20 mm horizontal aperture.

Referring to FIGS. 9-12C, the beam modification optical system 124 isarranged on a support 900 that is within a sealed housing 902. In thisimplementation, the highly reflective mirror 904 is manufactured to havea substantially flat profile in both the horizontal and the verticaldirections on its reflective surface 906 and is mounted to a bendingdevice (mirror mount) 908 that is arranged on the support 900. Themirror mount 908 acts to bend the mirror 904 to impart a convex shape tothe mirror 904. Similar to the mirror above, the highly reflectivemirror 904 is made upon a crystalline substrate or other substrate thatis robust against exposure to high average powers at very shortwavelengths, for example, at wavelengths of 193 nm or 248 nm. Forexample, the highly reflective mirror 904 can be formed upon a calciumfluoride (CaF2) or a magnesium fluoride (MgF2) substrate. Thereflectivity of the surface 906 can be greater than 94% for a raystriking the surface at 45° and at a wavelength of about 193 nm. Asshown in FIGS. 12A-C, in this design, the highly reflective mirror 904has a flat profile in both the horizontal direction and the verticaldirection.

Referring to FIGS. 11A-D, the mirror mount 908 bends the mirror 904along the horizontal direction such that the reflective surface 906 hasa convex shape along the horizontal direction. The mirror mount 908includes a rear device 1100 attached to a back side of a mirrorextension 1102 of the support 900 and a front device 1104 attached to afront side of the mirror extension 1102. The rear device 1100 includesone or more press devices 1106 that make contact with the rear of themirror 904 and the front device 1104 includes one or more press devices1108 that make contact with the front (the reflective surface 906) ofthe mirror 904. The mirror 904 is placed inside of an opening 1110formed in the mirror extension 1102 and between the press devices 1106and 1108. In operation, the rear device 1100 applies pressure to themirror 904 at locations where the one or more press devices 1106 contactthe rear of the mirror 904 and the front device 1104 applies pressure tothe mirror 904 at locations where the one or more press devices 1108make contact with the front of the mirror 904 to impart a convex shapeto the mirror 904.

As discussed above, the negative wavefront curvature system can beintegrated with any of the components of the optical system 124, forexample, the negative wavefront curvature system can be a modificationto the design and/or location of one or more components of the opticalsystem 124. In the example described above, the negative wavefrontcurvature system is implemented by modifying the highly reflectivemirror 204 to have a curved (convex) reflective surface.

As another example, and with reference to FIG. 4, the beam expansionsystem can be implemented by modifying one or more aspects of the prisms206, 208, and 214 to magnify the laser beam exiting the wavefrontmodification optical system 124 relative to the laser beam entering thewavefront modification optical system 124. For example, the relativedistance between the first and third prisms could be modified or therelative distance between the third and second prisms could be modified.As another example, the angle of placement of one or more of the prismscould be modified relative to the other prisms. As a further example,the material properties or surface figure of one or more of the prismscould be modified to change the de-magnification or magnification of thetransverse extent of the laser beam that passes through the prisms.

Thus, as shown in FIGS. 13A and 13B, and as discussed above, the firstand third prisms 206 and 208 are positioned and arranged relative toeach other so that they reduce the size of the transverse profile of thelaser beam 110 travelling along a first direction through the wavefrontmodification optical system 124. That is, the first and third prisms 206and 208, in combination and in the geometric configuration shown,de-magnify the horizontal size of the laser beam that travels along thefirst direction, which is from the highly reflective mirror 204 towardthe right chamber window 210, to substantially match the transverse sizeof the discharge plasma and effectively utilize the laser gain. Thethird and second prisms 208 and 214 are positioned and arranged relativeto each other so that they increase the size of the transverse profileof the laser beam 130 travelling along a second direction through thewavefront modification optical system 124. That is, the third and secondprisms 208 and 214, in combination and in the geometric configurationshown, magnify in the near-field the horizontal size of the laser beamthat travels along the second direction, which is from the right chamberwindow 210 to the input/output coupler 202, to match the transverseextent of the incoming laser beam 110 and/or the desired horizontal sizeof the output laser beam 130.

The net effect of the horizontal prism sequence (prisms 206 to 208 andthen prisms 208 to 214) is to slightly magnify the horizontal near-fieldsuch that the near field profile of the laser beam uniformly fills allof the apertures within the laser system 100 including apertures withinthe power ring amplifier 112 and the system aperture, and such that theregenerative ring resonator remains either conditionally stable ormarginally unstable when operating the laser at powers that inducethermal lenses in the optical elements inside of the regenerative ringresonator. To impart this net beam magnification (or expansion), onecould modify the relative angle between the third prism 208 and thesecond prism 214 to cause a slightly greater magnification in thehorizontal size of the laser beam that travels along the seconddirection. For example, as shown in FIG. 13B, one could do this byrotating the second prism 214 relative to the third prism 208 about anangle in the direction of the arrow 1320. The angle of rotation dependson how much greater magnification would be desired. For example, theangle of rotation could be greater than 0° (zero) but less than about10° relative to the position of the second prism 214 in the set up thatmatches the transverse size of the laser beam 130 with the transversesize of the laser beam 110. In order to rotate the prism 214, the prism214 can be mounted on a rotationally movable mount that is connected toa rotational position actuator such as a stepper motor and/or apiezoelectric-based actuator. In other implementations, one or more ofthe prisms 206, 208, and 214 are rigidly affixed so as to impart a fixedmagnification on each round trip.

Additionally, it is possible that the negative wavefront curvaturesystem can be implemented by, in combination, modifying one or moreaspects of the prisms 206, 208, and 214 and by modifying the highlyreflective mirror 204 to have a curved (convex) reflective surface.

Referring also to FIG. 14, a graph of the energy density (or the squareof the electric field amplitude) of the laser beam 130 versus distancefrom center of the beam along a transverse direction (for example, thehorizontal direction) is shown for two different scenarios. In the firstscenario, the beam modification optical system 124 of the regenerativering resonator (the PRA 112) lacks a beam expansion system that is thesubject of this disclosure (such as the negative wavefront curvaturesystem discussed above); that is, in this scenario, for example, thehighly reflective mirror has a substantially flat curvature. Raw datawas taken in the first scenario and a curve 1400 was estimated to bestfit the raw data for the first scenario. In the second scenario, thebeam modification optical system 124 includes a beam expansion system(such as the negative wavefront curvature system discussed above, forexample, the highly reflective mirror has a convex curvature). Raw datawas taken in the second scenario and a curve 1450 was estimated to bestfit the raw data for the second scenario. Additionally, the graph showsan aperture 1490 within the laser system 100 through which the laserbeam 130 travels.

A near field laser beam profile can be said to “uniformly fill” anaperture if an intensity at an edge of the aperture is greater than somefraction of a peak intensity at a center of the aperture. In someimplementations, the near field laser beam profile can be said to“uniformly fill” the aperture if the intensity at the edge of theaperture is greater than about 10% or about 20% of the peak intensity atthe center of the aperture.

One can see that in the first scenario, there is a narrowing of theenergy distribution across the laser beam 130 in the horizontaldirection, that is, the energy distribution becomes more concentrated(in other words, the laser beam collapses) in the near field so that thenear field laser beam does not fill the aperture 1490 as uniformly. Inparticular, the energy density indicated by the curve 1400 at an edge1491 or 1492 of the aperture 1490 is less than about 4% of the energydensity indicated by the curve 1400 at a center 1493 of the aperture1490. Such beam narrowing is unwanted because it can lead to damage tooptical components in the resonator and downstream of the resonator dueto increases in the peak irradiance of, and lack of stability in thelaser beam 130 to use for lithography applications.

By adding the beam expansion system (for example, the negative wavefrontcurvature system), the laser beam 130 exhibits a more even horizontalenergy distribution (or profile) in the near-field and this reduces thepotential for damage to optical components as shown in the secondscenario. The beam expansion system spreads the near field energydistribution of the laser beam (that is, it keeps the near field laserbeam from collapsing) so that the near field laser beam uniformly fillsthe aperture 1490. In particular, the intensity of the curve 1400 at theedge 1491 or 1492 is about 27% of the intensity of the curve 1400 at thecenter 1493 of the aperture 1490.

Other implementations are within the scope of the following claims. Forexample, the negative wavefront curvature system can be formed by addinga negative curvature optical device to the power ring amplifier 112 orby modifying one or more of the other optical components within thepower ring amplifier 112.

What is claimed is:
 1. A laser comprising: a regenerative ring resonatorcomprising: a discharge chamber having electrodes and a gain mediumbetween the electrodes; an optical coupler that is partially reflectiveso that at least a portion of a laser beam impinging on the opticalcoupler from the discharge chamber is reflected back through thedischarge chamber and at least a portion of the laser beam impinging onthe optical coupler from the discharge chamber is transmitted throughthe optical coupler; and a beam modification optical system in the pathof the laser beam and configured to impart a negative curvature to thewavefront of a laser beam that propagates in the regenerative ringresonator; wherein the beam modification optical system transverselyexpands a profile of the laser beam such that the near field laser beamprofile uniformly fills each aperture within the laser and such that theregenerative ring resonator remains either conditionally stable ormarginally unstable when operating the laser at average output powersthat induce thermal lenses in optical elements inside the regenerativering resonator.
 2. The laser of claim 1, wherein the beam modificationoptical system is between the optical coupler and a beam turning opticalelement placed on a side of the discharge chamber opposite to a sidefacing the optical coupler.
 3. The laser of claim 1, wherein the beammodification optical system is configured to cause the laser beam at theexit of the regenerative ring resonator to have the same or a largersize of the transverse beam profile than the size of the transverse beamprofile of the laser beam entering the regenerative ring resonator. 4.The laser of claim 1, wherein the beam modification optical system isconfigured to impart a negative curvature to the wavefront of the laserbeam circulating within the regenerative ring resonator.
 5. The laser ofclaim 4 1, wherein the beam modification optical system negativelyalters the curvature along a transverse direction.
 6. The laser of claim4 1, wherein the beam modification optical system comprises a highlyreflective mirror.
 7. The laser of claim 6, wherein the optical systemincludes the highly reflective mirror that is convex.
 8. The laser ofclaim 7, wherein the convex highly reflective mirror has a radius ofcurvature of between about 50 m and about 170 m.
 9. The laser of claim1, further comprising a beam turning optical element external to thedischarge chamber and in the path of the laser beam on a side of thedischarge chamber that is opposite to a side that faces the opticalsystem.
 10. The laser of claim 1, wherein the regenerative ringresonator remains marginally unstable if the size of the transverseprofile of the laser beam increases as the laser beam travels through aportion of the regenerative ring resonator but the laser beam transverseprofile size does not exceed the transverse size of any of the opticalcomponents within the regenerative ring resonator before being decoupledout of the regenerative ring resonator through the optical coupler. 11.The laser of claim 1, wherein the beam modification optical systemcomprises a set of prisms.
 12. The laser of claim 11, wherein the prismset comprises first, second, and third prisms configured and arranged sothat the first and third prisms reduce the transverse size of theprofile of the laser beam travelling along a first direction through thebeam modification optical system, and the third and second prismsincrease the transverse size of the profile of the laser beam travellingalong a second direction through the beam modification optical system.13. The laser of claim 12, wherein the one or more first, second, andthird prisms are adjusted so that the transverse size of the profile ofthe laser beam travelling along the second direction through the beammodification optical system and exiting the prism set is greater thanthe transverse size of the profile of the laser beam travelling alongthe first direction from the optical coupler to the prism set.
 14. Thelaser of claim 1, wherein the laser beam output from the regenerativering resonator has an average irradiance of at least about 5 W/cm². 15.The laser of claim 1, wherein the laser beam output from theregenerative ring resonator has an irradiance of at least about 10W/cm².
 16. A method of modifying a laser beam of an electric dischargegas laser, the method comprising: directing a laser beam through anoptical coupler of a regenerative ring resonator; directing the laserbeam that passes through the optical coupler through a discharge chamberand back to the optical coupler such that at least some of the lightimpinging on the optical coupler from the discharge chamber is reflectedback through the discharge chamber and at least some of the lightimpinging on the optical coupler from the discharge chamber istransmitted through the optical coupler; and transversely expanding aprofile of the laser beam such that the near field laser beam profileuniformly fills each aperture within the laser and such that theregenerative ring resonator remains either conditionally stable ormarginally unstable when operating the laser at powers that causethermal leasing of elements within the regenerative ring resonator;wherein transversely expanding the profile of the laser beam includesimparting a negative curvature to the wavefront of a laser beam thatpropagates in the regenerative ring resonator.
 17. The method of claim16, wherein transversely expanding the profile of the laser beamcomprises: compressing a profile of the laser beam by passing the laserbeam from the optical coupler through first and third prisms of a prismset before directing the laser beam through the discharge chamber; andexpanding a profile of the laser beam after it has passed through thedischarge chamber by passing the laser beam through the third prism andthrough a second prism of the prism set before reaching the opticalcoupler.
 18. The method of claim 17, wherein expanding the laser beamprofile includes expanding the laser beam profile to a size that islarger than the profile of the laser beam input to the prism set. 19.The method of claim 16, wherein the regenerative ring resonator remainsmarginally unstable if the size of the transverse profile of the laserbeam increases as the laser beam travels through at least a portion theregenerative ring resonator but the laser beam transverse profile sizedoes not exceed the transverse size of any of the optical componentswithin the regenerative ring resonator before being decoupled out of theregenerative ring resonator through the optical coupler.
 20. The methodof claim 16, wherein transversely expanding the profile of the laserbeam includes imparting a negative curvature to the wavefront of thelaser beam circulating within the regenerative ring resonator.
 21. Aregenerative ring resonator in the path of a laser beam, the resonatorcomprising: a discharge chamber having electrodes and a gain mediumbetween the electrodes; an optical coupler that is partially reflectiveso that at least some of the light impinging on the optical coupler fromthe discharge chamber is reflected back through the discharge chamberand at least some of the light impinging on the optical coupler from thedischarge chamber is transmitted through the optical coupler; and a beammodification optical system in the path of the laser beam; wherein thebeam modification optical system is configured to transversely expand aprofile of the laser beam such that the near field laser beam profileuniformly fills each aperture within the resonator and such that theregenerative ring resonator remains either conditionally stable ormarginally unstable when operating the laser at powers that causethermal lensing of elements inside of the regenerative ring resonator;wherein the beam modification optical system is configured to impart anegative curvature to the wavefront of a laser beam propagating in theregenerative ring resonator.
 22. The resonator of claim 21, wherein thebeam modification optical system is between the optical coupler and abeam turning optical element placed on a side of the discharge chamberopposite to a side facing the optical coupler.
 23. The resonator ofclaim 21, wherein the beam modification optical system is configured toimpart a negative curvature to the wavefront of the laser beamcirculating within the regenerative ring resonator.
 24. The resonator ofclaim 23 21, wherein the beam modification optical system comprises ahighly reflective convex mirror.
 25. The resonator of claim 21, whereinthe regenerative ring resonator remains marginally unstable if the sizeof the transverse profile of the laser beam increases as the laser beamtravels through at least a portion of the regenerative ring resonatorbut the laser beam transverse profile size does not exceed thetransverse size of any of the optical components within the regenerativering resonator before being decoupled out of the regenerative ringresonator through the optical coupler.
 26. The resonator of claim 21,wherein the beam modification optical system comprises a set of prisms.27. The resonator of claim 26, wherein the prism set comprises first,second, and third prisms configured and arranged so that the first andthird prisms reduce the transverse size of the profile of the laser beamtravelling along a first direction through the beam modification opticalsystem, and the third and second prisms increase the transverse size ofthe profile of the laser beam travelling along a second directionthrough the beam modification optical system.
 28. The resonator of claim27, wherein the one or more first, second, and third prisms are adjustedso that the transverse size of the profile of the laser beam travellingalong the second direction and exiting the prism set is greater than thetransverse size of the profile of the laser beam travelling along thefirst direction from the optical coupler to the prism set.
 29. The laserof claim 1, wherein the laser beam output from the regenerative ringresonator has an average output power of greater than 40 W.
 30. Aregenerative ring resonator comprising: a discharge chamber havingelectrodes and a gain medium between the electrodes; an optical couplerthat is partially reflective so that at least a portion of a laser beamimpinging on the optical coupler from the discharge chamber is reflectedback through the discharge chamber and at least a portion of the laserbeam impinging on the optical coupler from the discharge chamber istransmitted through the optical coupler; and a beam modification opticalsystem in the path of the laser beam between the optical chamber and theoptical coupler, the beam modification optical system includes a mirrorthat transversely expands a profile of the laser beam by negativelyaltering a wavefront curvature of the laser beam so that a divergence ofthe laser beam is increased along at least one transverse direction. 31.The resonator of claim 30, wherein the mirror is a highly reflectivemirror.
 32. The resonator of claim 31, wherein the highly reflectivemirror is convex along the direction of transverse expansion.
 33. Theresonator of claim 31, wherein the highly reflective mirror has asurface reflectivity that is greater than 94%.
 34. The resonator ofclaim 30, wherein the beam modification optical system also includes aset of prisms.
 35. The resonator of claim 30, further comprising a beamturning optical element external to the discharge chamber and in thepath of the laser beam on a side of the discharge chamber that isopposite to a side that faces the beam modification optical system. 36.A regenerative ring resonator comprising: a discharge chamber havingelectrodes and a gain medium between the electrodes; an optical couplerthat is partially reflective so that at least a portion of a laser beamimpinging on the optical coupler from the discharge chamber is reflectedback through the discharge chamber and at least a portion of the laserbeam impinging on the optical coupler from the discharge chamber istransmitted through the optical coupler; and a beam modification opticalsystem in the path of the laser beam between the optical chamber and theoptical coupler, the beam modification optical system includes anoptical component that transversely expands a profile of the laser beamby negatively altering a wavefront curvature of the laser beam so thatthe divergence of the laser beam is increased along at least onetransverse direction; wherein the profile of the laser beam istransversely expanded such that the size of the transverse profile ofthe laser beam increases as the laser beam travels through at least aportion of the regenerative ring resonator but the laser beam transverseprofile size does not exceed the transverse size of any of the opticalcomponents within the regenerative ring resonator before being decoupledout of the regenerative ring resonator through the optical coupler. 37.A regenerative ring resonator comprising: a discharge chamber havingelectrodes and a gain medium between the electrodes; an optical couplerthat is partially reflective so that at least a portion of a laser beamimpinging on the optical coupler from the discharge chamber is reflectedback through the discharge chamber and at least a portion of the laserbeam impinging on the optical coupler from the discharge chamber istransmitted through the optical coupler; and a beam modification opticalsystem in the path of the laser beam between the optical chamber and theoptical coupler, the beam modification optical system includes anoptical component that transversely expands a profile of the laser beamby negatively altering a wavefront curvature of the laser beam so thatthe divergence of the laser beam is increased along at least onetransverse direction; wherein the size of the transverse profile of thelaser beam increases as the laser beam travels through a portion of theregenerative ring resonator but the laser beam transverse profile sizedoes not exceed the transverse size of any of the optical componentswithin the regenerative ring resonator before being decoupled out of theregenerative ring resonator through the optical coupler.
 38. Aregenerative ring resonator of claim 37, wherein the beam modificationoptical system also includes: a plurality of prisms arranged between theoptical coupler and the discharge chamber and the plurality of prismscomprises first, second, and third prisms configured and arranged sothat the first and third prisms reduce the transverse size of theprofile of the laser beam travelling along a first direction through thebeam modification optical system, the first direction entering the beammodification optical system, and the third and second prisms increasethe transverse size of the profile of the laser beam travelling along asecond direction through the beam modification optical system, thesecond direction exiting the beam modification optical system.